Ras proteins are small GTPases that act as signal transducers between cell surface receptors and several intracellular signaling cascades. These molecules regulate such essential cellular functions as cell survival, proliferation, motility, and cytoskeletal organization (see Karnoub et al., Nat. Rev. Mol. Cell Biol., 9: 517-531 (2008)).
Ras proteins function as GDP/GTP-regulated binary switches in signal transduction cascades that can lead to cell growth, proliferation, differentiation, or survival. In its active form, Ras is bound to GTP. This causes a conformational change that allows it to interact and bind to several effector molecules, most notably the members of the Raf family, the RalGDS family, and Phosphoinositide 3-kinases (PI3 Kinase). Ras then cleaves GTP to GDP resulting in its inactivation. In its oncogenic, mutated state, Ras is unable to hydrolyze GTP to GDP, thus staying in an active state and activating numerous pathways.
The Ras superfamily has at least five major branches that include Ras, Rho, Ran, Arf/Sar, Rab. The four classical p21 Ras proteins are H-Ras (Harvey sarcoma viral oncogene), N-Ras (neuroblastoma oncogene), and the splice variants K-Ras4A and K-Ras4B (Kirsten sarcoma viral oncogene) (see Karnoub et al., supra). They are collectively referred to as Ras.
The p21 Ras proteins share 85% of sequence homology and activate very similar signaling pathways. However, recent studies clearly demonstrate that each Ras isoform functions in a unique, radically different way from the other Ras proteins in normal physiological processes as well as in pathogenesis (Quinlan et al., Future Oncol., 5: 105-116 (2009)). According to Catalogue of Somatic Mutations in Cancer (www.sanger.ac.uk/genetics/CGP/cosmic/), K-Ras mutations were detected in 22.1% of analyzed human tumors, N-Ras in 8.2%, and H-Ras in 3.3%.
Mutations in cellular Ras have been found to be present in a large percentage of all human cancers, such as leukemias, colon cancers, and lung cancer (see Quinlan et al., supra, and Boissel et al., Leukemia, 20: 965-970 (2006)). More specifically, K-Ras mutations occur frequently in lung, pancreatic, and colon cancers, where as H-Ras mutations are prevalent in bladder, kidney, thyroid, and salivary gland cancers (see Shulz, Int. J. Cancer, 119: 1513-1518 (2006), and Yoo et al., Arch. Pathol., Lab Med., 124: 836-839 (2000)), and N-Ras mutations are associated with myeloid malignancies, germ cell tumors, melanoma, hepatocellular carcinoma, and leukemia. Additionally, K-Ras mutation is predictive of response to EGFR antagonists therapy in colorectal cancer (see Lopez-Chavez et al., Curr. Opin. Investig. Drugs, 10: 1305-1314 (2009)).
Despite the central role of ras proteins in oncogenesis and wide-spread efforts to develop ras-directed anti-cancer therapeutics, no selective, specific inhibitor of the ras pathway is available for clinical use, and ras mutant cancers remain among the most refractory to available treatments (Adjei, J. Thorac. Oncol., 3: S160-163 (2008); Graham et al., Recent Results Cancer Res., 172: 125-153 (2007); and Saxena et al., Cancer Invest., 26: 948-955 (2008)). Moreover, no inhibitors acting directly on ras oncogenes have been developed even for in vitro use. Consequently, ras proteins are considered to be non-druggable targets. Therefore, there is a desire to identify inhibitors of ras oncoproteins.